Britain's High Court has brought government plans for leaving the European Union screeching to a halt, ruling that the prime minister can't trigger the UK's exit from the bloc without approval from Parliament.

LISTEN ABOVE: UK correspondent Ben Bland speaks with Rachel Smalley

The government says it will go to the Supreme Court to challenge the ruling, which has major constitutional as well as practical implications.

The pound, which has lost about a fifth of its value since the June decision to leave, shot up on the verdict, rising 1.1 per cent to $US1.24.

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Britons voted by a margin of 52 to 48 per cent to leave the EU, a process known as "Brexit." Several claimants challenged the plans for Brexit in a case hinging on the balance of power between Parliament and the government.

Prime Minister Theresa May has said she will launch exit negotiations with the EU by March 31. She is relying on a power called the royal prerogative that lets the government withdraw from international treaties.

Claimants argue that leaving the EU will remove rights, including free movement within the bloc, and say that can't be done without Parliament's approval.

Three senior judges ruled that "the government does not have the power under the Crown's prerogative" to trigger the official exit process.

The British government immediately said it would appeal the judgment. The government said in a statement that Britons voted to leave the bloc in a referendum approved by an Act of Parliament, "and the government is determined to respect the result of the referendum."

• Gina Miller: Woman who put brakes on Brexit

The Supreme Court has set aside time to hear the appeal before the end of the year.

The case is considered the most important constitutional matter in a generation.

Financial entrepreneur Gina Miller, a lead claimant in the case, said the result "is about all of us ... It's about our United Kingdom and all our futures."

She backed the losing "remain" side in the EU referendum, but has said the lawsuit is not an attempt to stop Brexit - just to ensure that Parliament is sovereign.

Still, the pound's rise signaled that the ruling boosted the hopes of the financial sector, which is largely opposed to Brexit.

The ruling has angered pro-Brexit campaigners, who fear politicians might try to block or delay Britain's EU exit. UK Independence Party leader Nigel Farage, who helped lead the campaign against the EU, tweeted: "I worry that a betrayal may be near at hand."